This invention relates to a system for feedback control of the air/fuel ratio in an internal combustion engine, particularly an automotive engine, by using an oxygen sensor as an exhaust gas sensor to detect actual values of the air/fuel ratio. The control system includes means to interrupt the feedback control and use open-loop control during transient acceleration or deceleration of the engine.
In the current automotive internal combustion engines, it is popular to perform feedback control of the air/fuel ratio to meet the exhaust emission regulations and also to meet the growing demands for better fuel economy and improved driveability. Usually an oxygen sensor comprising a solid electrolyte cell is used to estimate actual values of the air/fuel ratio in the engine from the concentrations of oxygen in the exhaust gas, and a signal representative of the deviation of the actual air/fuel ratio from the target value is produced in an electronic control unit. By, for example, proportional and/or integral treatment of the air/fuel ratio deviation signal, a feedback signal to control the functions of a fuel-feed or air-fuel proportioning device such as carburetor or fuel injector is produced.
In the cases of automotive engines using fuel injectors, for example, a standard amount of fuel injection T.sub.p is varied according to the engine operating conditions and may be given by the equation (1): EQU T.sub.p =K.(Q.sub.a /N) (1)
wherein K is a constant, Q.sub.a is the flow rate of air being taken into the engine, and N is the revolving speed of the engine.
When feedback control of the air/fuel ratio is performed, a corrected amount of fuel injection T.sub.i is computed by using a feedback signal to cancel deviations of the actual air/fuel ratio from the target value. For example, T.sub.i is given by the equation (2): EQU T.sub.i =T.sub.p .times.C.sub.f .times..alpha.+T.sub.s ( 2)
where C.sub.f is a weighting factor which is variable depending on some parameters of the engine operating conditions such as the temperature of the cooling water, degree of opening of the throttle valve, etc., .alpha. is a feedback correction factor computed by the aforementioned proportional and/or integral treatment of an air/fuel ratio deviation signal, and T.sub.s is a correction factor for compensation of a delay in the response of the fuel injector to a control or command signal.
When starting an automotive engine provided with a conventional air/fuel ratio control system it is customary to defer the start of the feedback control operation until completion of warm-up of the engine. This is because conventional oxygen sensors do not accurately function when the temperature is too low and also when the air/fuel ratio changes over a wide range as is frequent during warm-up of the engines. Recently several types of oxygen sensors which are, as exhaust gas sensors, responsive to changes in the air/fuel ratio in the engine over a fairly wide range including both sub-stoichiometric and superstoichiometric regions have been developed. Accordingly, a recent trend is to commence the feedback control of the air/fuel ratio soon after starting the engine by using a recently developed oxygen sensor, which is provided with a heater, with a view to satisfying the demands for better fuel economy, improved driveability and more strict control of exhaust emission. For example, U.S. patent application Ser. No. 726,586 of the common assignee, filed Apr. 23, 1985, shows an air/fuel ratio feedback control system of this category.
In many of conventional and hitherto proposed air/fuel ratio feedback control systems using an oxygen sensor, feedback control of the air/fuel ratio is temporarily shifted to open-loop control under transient operating conditions of the engine, because sudden and great changes in the amounts of air and fuel fed to the engine under steep acceleration or deceleration are not quickly and accurately reflected in the air/fuel ratio detected by the oxygen sensor. The duration of the temporary open-loop control is a predetermined constant. When the engine temperature is sufficiently high the constantness of the duration of the temporary open-loop control offers little problem because accurate response of the oxygen sensor is resumed in an almost constant time irrespective of engine temperature and the type of the transient operating condition. However, if the engine has not sufficiently warmed up it is likely that the predetermined duration of the temporary open-loop control is too short or too long. A fundamental reason is that during warm-up of the engine the vaporization of fuel remains incomplete so that more than a negligible quantity of fuel in liquid state adheres to and flows on the wall surfaces in the intake manifold and intake ports. When the engine under warm-up is steeply accelerated or decelerated, a discrepancy between a calculational air/fuel ratio established by controlled operation of the air-fuel proportioning device and an actually effective air/fuel ratio at which combustion takes place and which can be detected by the oxygen sensor becomes greater than in the cases of transient operations after warming up. Furthermore, under transient operating conditions the length of a time period for which the detection of air/fuel ratio by the oxygen sensor is unreliable becomes longer as the engine temperature is lower. For these reasons, it is often that in conventional air/fuel ratio feedback control systems the temporary open-loop control of air/fuel ratio is shifted to feedback control before proper response of the oxygen sensor is resumed or is continued for an unnecessarily long time.